Messages in this thread | | | Date | Fri, 11 Dec 2015 17:26:48 -0600 | From | Chris Friesen <> | Subject | Re: question about cpusets vs sched_setaffinity() |
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On 12/11/2015 04:15 PM, Jason Baron wrote: > On 12/10/2015 04:30 PM, Chris Friesen wrote:
>> If I put a task into a cpuset and then call sched_setaffinity() on it, >> it will be affined to the intersection of the two sets of cpus. (Those >> specified on the set, and those specified in the syscall.) >> >> However, if I then change the cpus in the cpuset the process affinity >> will simply be overwritten by the new cpuset affinity. It does not seem >> to take into account any restrictions from the original >> sched_setaffinity() call. >> >> Wouldn't it make more sense to affine the process to the intersection >> between the new set of cpus from the cpuset, and the current process >> affinity? That way if I explicitly masked out certain CPUs in the >> original sched_setaffinity() call then they would remain masked out >> regardless of changes to the set of cpus assigned to the cpuset.
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> To add the behavior you are describing, I think requires another > cpumask_t field in the task_struct. Where we could store the last > requested mask value for sched_setaffinity() and use that when updating > the cpus for a cpuset via an intersection as you described. I think > adding a task to a cpuset still should wipe out any sched_setaffinity() > settings - but that would depend on the desired semantics here. It would > also require a knob so as not to break existing behavior by default.
Agreed, the additional field in the task_struct makes sense. Personally I don't think that adding a task to a cpuset should wipe out any previously-set affinity, I think it should take the intersection for that case as well.
In this environment it might make sense to have separate queries to return the requested and actual affinity.
> You could also create a child cgroup for the process that you don't want > to change and set the cpus on that cgroup instead of using > sched_setaffinity(). Then you change the cpus for the parent cgroup and > that shouldn't affect the child as long as the child cgroup is a subset. > But its not entirely clear to me if that addresses your use-case?
I ended up doing something like this where I had a top-level cpuset and a number of child cpusets, each with an exclusive subset of the CPUs assigned to it. But it meant that I needed more complicated code to figure out which tasks needed to go into which child cpusets, and more complicated code to handle removing a CPU from the top-level cpuset (since you have to remove it from any children first).
Chris
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